Absorption refrigeration, chilling, heat pump, energy storage, and other heat transferring apparatus employs a refrigerant/absorbent composition to accept heat from a heat load and transfer this heat to a suitable place for rejection. The refrigerant/absorbent composition includes a more-volatile refrigerant component (such as water) and a less-volatile absorbent component (such as a concentrated aqueous salt solution).
A typical refrigeration loop has four basic components--a generator, a condenser, an evaporator, and an absorber. In the operation of this loop, the refrigerant component is distilled out of the refrigerant/absorbent composition in the generator. After this distillation, the absorbent component left behind is removed to the absorber. At the same time, the distilled refrigerant vapor moves to the condenser where it condenses and sheds heat. The refrigerant liquid then proceeds to the evaporator which is in contact with the heat load for which cooling is desired. The refrigerant absorbs heat and vaporizes once again in the evaporator. The refrigerant vapor is then removed to the absorber where it sheds heat when it is reabsorbed by the absorbent component. The recombined refrigerant/absorbent composition is returned to the generator to complete one cycle. Absorption heat transferring apparatus and its operation are described in greater detail in U.S. Pat. No. 5,284,029, issued Feb. 8, 1994, and assigned to the Gas Research Institute. That patent is incorporated by reference in its entirety here.
In a triple effect absorption apparatus, two of the refrigeration loops described in the preceding paragraph having higher and lower respective temperatures are combined. These two loops operate so that the absorber and condenser of the higher temperature loop are in heat exchange with the generator of the lower temperature loop. Triple effect absorption apparatus and its operation are described in greater detail in U.S. Pat. No. 4,732,008. That patent is hereby incorporated by reference in its entirety into the present disclosure.
The higher temperature loop of the triple-effect absorption apparatus operates in much the same fashion as a conventional single-effect refrigeration loop, except that the condenser and absorber operate at a much higher temperatures than the corresponding components of the lower temperature loop. The two refrigeration loops of the triple-effect apparatus typically contain different refrigerant/absorbent compositions which are independently selected for the particular conditions of one loop.
One refrigerant/absorbent composition frequently used in absorption heat transferring apparatus is a concentrated aqueous composition of lithium bromide (and sometimes other water-soluble salts) as the absorbent, and additional water as the refrigerant. Other ingredients which may be present include lithium chloride and zinc and calcium bromides and chlorides. Such compositions are disclosed generally in U.S. Pat. No. 3,478,530, which is incorporated here by reference.
Typically, water/lithium bromide solutions which do not contain additional ingredients are not practical for the high temperature loop of a triple effect absorption heat transfer machine, due to their solubility and vapor pressure. A single salt aqueous refrigerant/absorbent composition of lithium bromide is not soluble enough to provide a desirably low vapor pressure of the working fluid (about 10 mmJg) at the high absorber temperatures, e.g. about 190.degree.-240.degree. F. (87.degree.-115.degree. C.), required in the high temperature refrigeration loop.
Absorption compositions that can meet these temperature, pressure and solubility requirements include aqueous solutions of lithium, zinc, and calcium bromides and chlorides disclosed generally in U.S. Pat. No. 3,478,530, which is incorporated by reference in its entirety here.
One difficulty which arises from the use of aqueous solutions of bromides, chlorides, and other salts as working fluids is corrosion of the metal parts of the absorption refrigeration apparatus. Corrosion can be a particularly acute problem when the apparatus has steel or copper parts (which are the predominant materials used for piping and heat transfer surfaces). Corrosion decreases the operable lifetime of an apparatus, can reduce its heat transfer capacity, and may erode one or more internal surfaces of an hermetically sealed apparatus to the point of causing leakage of the refrigerant/absorbent composition. Corrosion poses a further problem by forming by-products such as hydrogen gas within the absorption refrigeration system. Hydrogen is a noncondensible gas at the working temperature of the apparatus and thus interferes with the normal operation of the system.
Additives and combinations of additives have been considered before to reduce the corrosive effects of refrigerant/absorbent compositions on metal parts and surfaces. For example, U.S. Pat. No. 4,470,272 proposes molybdates, chromates, nitrates, and tungstates as corrosion-inhibiting additives in lithium bromide-water compositions for triple-effect absorption refrigerators. Although the prior art suggests that these additives display some degree of effectiveness in lessening the corrosion of ferrous metal parts, some or all of these additives do not sufficiently inhibit the corrosive effects of aqueous compositions of lithium and zinc bromides or lithium, zinc and calcium bromides.
The '272 patent also proposes organic inhibitors such as 1,3-diethyl-2-thiourea, thiosinamine, sodium thiocyanate, diphenylthiocarbazide, and 1-acetyl-2-thiourea in lithium bromide-water compositions. The '272 patent states that these inhibitors are suitable for lithium bromide-water compositions in single-effect refrigerators. The '272 patent does not teach the use of these inhibitors in the higher temperature loop of a triple-effect absorption refrigerator, or that these organic inhibitors are effective with compositions other than lithium bromide and water.